The wavefront error of the human eye is analogous to the surface of a drum. The diameter of the drum represents the diameter of the pupil of the eye. If there were no aberrations, the surface of the drum would be flat. However, in almost any normal eye there are optical errors represented by leads or lags in the wavefront, and in the analogy, hills and valleys in the drum surface. Any smooth surface over a circular aperture can be described as a sum of coefficients multiplied by Zernike polynomials. Optical aberrations in the human eye are now almost exclusively described in this manner. The lowest order Zernike aberrations are shown in FIG. 1. Not shown in the “Zernike pyramid” are the first two lines (radial orders) which include the Zernikes piston, tip and tilt. Conventional glasses include prism, sphere and cylinder correction. Prism is simply tip and tilt, while sphere and cylinder are linear combinations of defocus and astigmatism.
Shown in FIG. 1 are the 2nd to 4th order Zernikes.1 Astigmatism, sphere, tip & tilt are considered “low-order” aberrations. All other aberrations of higher radial order are known collectively as “high-order” aberrations. Conventional glasses attempt to provide the best possible low-order correction.
Average pupil size significantly decreases with age. Persons of age 18 yrs-40 yrs have pupil diameters in the neighborhood of 6.5 mm for lighting conditions typical of evening on an overcast day (44 cd/m2).2,3 Average high-order root-mean-square (RMS) wavefront error in a 6.5 mm pupil is of the order of 0.38 microns.4 Many people have greater levels of high-order aberration. For comparison, to achieve a similar RMS wavefront error in a 3 mm daytime pupil requires a 1.2-diopter sphere error, which is considered large. Whereas residual sphere error can be nullified via the variable focusing power of the lens through the process of accommodation, the eye has no mechanism for changing the amount of high-order aberrations.
Visual acuity characterizes the ability to resolve small objects. Acuity measures only a portion of visual ability. It is, however, one of the more well known measures of vision. Population averages of visual acuity versus age for normal healthy eyes are summarized in FIG. 2 which shows the log MAR VA of 223 subjects ranging from 18 to 80 years of age. The best linear and bilinear fits to the data are shown.5 The subjects used the best possible conventional low-order correction for the measurements.
In the graph of FIG. 2, 6/6 is the metric equivalent of 20/20. 6/3 is the metric equivalent of 20/10 and represents approximately the predicted Nyquist resolution limit due to the cone density in the human fovea.6 Theoretically, if there were no optical aberrations, the human eye should be capable of seeing approximately 20/10, although the exact value probably varies from person to person. Until recently, the world's record was 20/8. Young people of age 25-29 typically have the best acuity, and vision generally worsens with increasing age after about age 30. The reason for the decline with age is a topic of debate. The three main theories include increases in high-order aberrations and concomitant reduction in pupil size, increases in intraocular scatter and transmission loss, and loss of cones and/or ganglion cells. Published reports tend to support the first two theories for normal healthy eyes.
The reason for the scatter in the data at any given age, especially for the younger eyes, is probably mostly due to the presence of high-order aberrations. One theory is that if a person was never exposed to good vision when young, the neural development may preclude seeing at or near the Nyquist resolution limit later in life, a condition referred to as refractive amblyopia.7 Nevertheless, when high-order aberrations are corrected using adaptive optics, the visual acuity of all subjects significantly improves8. In a recent study, half of the high-order-corrected subjects consistently demonstrated an acuity exceeding 20/8.9 Therefore, vision benefit from correction of high-order aberrations is not just theoretical.
One of the largest anticipated benefits of high-order aberration correction is an improvement in contrast sensitivity. In low-light conditions when the pupil diameter and the level of high-order aberrations both increase, contrast sensitivity begins to degrade. This has two deleterious effects. One is that it may no longer be possible to detect certain low-contrast objects which are important in for example driving, hunting and military applications. The very definition of camouflage is to reduce contrast by better matching the surrounding conditions. The other problem is that even if an object can be detected, the detection and recognition process will take longer. Studies consistently show that reaction times are increased when contrast sensitivity is degraded.10,11 
The three major higher-order aberrations affecting typical people include coma, trefoil and spherical aberration. Coma is a non-symmetric aberration capable of causing significant loss of contrast sensitivity. FIG. 3 shows a high-contrast eye-chart optical simulation for a subject with 0.19 microns RMS of coma in a 6.0 mm pupil.12 This is within a standard deviation of the average value.13 The simulation assumes that all other aberrations are fully corrected. The top line is 20/100.
In a low contrast situation, the letters would be even more difficult to recognize, and of course, there are many other possible aberrations besides coma. For comparison, an un-aberrated (no aberrations) eye chart simulation based on a 6.0 mm pupil is shown in FIG. 4.
The modulation transfer function (MTF) characterizes how well an optical system preserves contrast versus spatial frequency. The MTF of a diffraction-limited eye having no aberrations with a 6.0 mm pupil is shown in FIG. 5.
The MTF graph with the introduction of just the 0.19 microns of coma in a 6.0 mm pupil is seen in FIG. 6.
The MTF curve is severely depressed at all spatial frequencies, due to the single aberration of coma. If sphere and cylinder are not also optimally corrected, there will in addition be severe contrast losses due to low-order aberrations.